It is for "fiction titles published for young adults in the past 16 months that are recommended reading for ages 12 to 18.
[1][2] It has undergone several changes of focus and names over the years, including the "Book Selection Committee" (1954), the "Committee for the Selection of Significant Adult Books for Young People" (1963).
[2] Books marketed for "young adults" have been considered since then and now constitute a majority of the selections.
Meanwhile, the marketing category has changed to include more books oriented to older teens.
[2] A list of historical recipients is available in Betty Carter, Best Books for Young Adults, 1st Edition (1994).